The present invention relates to an image creation apparatus for presenting images on a display device on which a plurality of picture elements, or pixels, are arranged in matrix form.
An example of this type of image creation apparatus is a personal computer complying with an industrial standard known as "MSX" (a trademark for a computer hardware and software standard). A video game image presented by this personal computer is essentially a group of dot patterns called "characters," (or dot pattern) each composed of 8.times.8 pixels, for instance. A video random access memory (hereinafter referred to as the VRAM) employed by the personal computer is of a so-called character-generation type suitable for displaying such characters.
As example of the character-generation type VRAM has physically partitioned first and second memory areas for storing, respectively, character image data which defines 256 kinds of dot patterns, or characters, and screen data which determines at which on-screen locations of a display device individual character images generated from the character image data are displayed. The character image data for creating up to 256 kinds of dot patterns and initial screen data are read from an external storage medium such as a read-only memory (hereinafter referred to as the ROM) cassette and written into the VRAM at the beginning of a game or at the end of a stage, or a particular sequence of events, during the game, for instance. When the screen data is updated by a control circuit in accordance with the progress of the game, characters (dot patterns) designated by the screen data are read from the VRAM and displayed at specified locations on a screen of the display device through a hardware-driven process. A virtual display screen created in the VRAM to cover the screen of the display device is divided into segments, each matching the size of a single character (or a single dot pattern). This means that on-screen images can only be rewritten in the unit of the character size.
An image creation apparatus proposed in recent years allows an operator to produce an image by using a coordinate input device like a tablet and to present the image on a screen of a display device such as a home television set, for instance. Since this image creation apparatus displays precise images including line drawings, it employs a VRAM suitable for such applications. Specifically, the VRAM used in the image creation apparatus has addresses corresponding to individual pixels of the display device and allows color specification for the individual addresses so that display colors can be specified for the individual pixels of the display device. This type of VRAM is referred to as a bit-map type VRAM.
When the bit-map type VRAM is used for presenting images on the display device, a control circuit is provided with a capability to write color specification data in individual addresses of the VRAM and switch display pages.
In the conventional image creation apparatus employing the bit-map type VRAM, it is essential to write full-screen pixel data in the VRAM to display a desired screen image. This means that an enormous amount of image data is required for displaying a wide variety of images or an extremely large-sized image on a screen. On the other hand, the image creation apparatus is limited in image data storage capacity since its program data and image data are supplied in the form of an IC memory card to ensure portability and ease of handling. It has therefore been difficult in the aforementioned image creation apparatus to present such a wide variety of images or an extremely large-sized image which exceeds the storage capacity of the IC memory card.